The intrapulmonary delivery of pharmaceutically active drugs is accomplished by a number of distinct methodologies. In accordance with one method, a pharmaceutically active drug is dispersed in a low boiling point propellant (a CFC or HFA) and loaded in a pressurized canister from which the drug/propellant formulation may be released by the use of a device generally known as a metered dose inhaler (MDI). Once released, the propellant evaporates and particles of the drug are inhaled by the patient. Another method involves the use of a nebulizer which creates a mist of fine particles from a solution or suspension of a drug which mist is inhaled through the mouth and/or nose by the patient. In yet another method of dry powered drug (which may be included in packets) are inhaled. These methods are hindered by significant problems relating to patient compliance and dosing as described further below.
The use of dry powders in systems presents some unique difficulties. Firstly, the dry powders are difficult to store and can be easily contaminated with water vapors causing the powders to clump together. Systems which do not include dry powders include the drug dissolved or suspended in a liquid carrier. Although there are advantages to these systems (e.g., avoiding the clumping of powder particles) these systems are also affected by moisture in the surrounding air, i.e. humidity. Specifically, such systems may use water as the carrier, i.e. a formulation comprised of a drug and water is used to create aerosolized particles. The carrier such as the water in the particles evaporates after the particles are formed. However, the rate and amount of evaporation can vary if the surrounding humidity varies. The amount of evaporation effects the particle size which effects the amount of particles which reach the lung and the particular area of the lung which the particles can reach. Several aspects of the present invention are directed at eliminating the variability in particle size due to surrounding humidity. Other devices known for intrapulmonary drug delivery are discussed below as are some of the advantages and disadvantages of these devices.
Metered dose inhalers that are generally manually operated and some breath actuated devices have been proposed and produced. Breath actuated inhalers typically contain a pressurized propellant and provide a metered dose automatically when the patient's inspiratory effort either moves a mechanical lever or the detected flow rises above a preset threshold, as detected by a hot wire anemometer. See, for example, U.S. Pat. Nos. 3,187,748; 3,565,070; 3,814,297; 3,826,413; 4,592,348; 4,648,393; 4,803,978; 4,896,832; and a product available from 3M Healthcare known as Aerosol Sheathed Actuator and Cap.
A major problem with manual metered dose inhalers is that the patient frequently actuates the device at the incorrect point during the breathing cycle to obtain the benefits of the intended drug therapy or breathes at the wrong flow rate. Thus, patients may inspire too little medication, or take additional doses and receive too much medication or deliver the drug to the throat or mouth rather than the lung. The problem is, therefore, the inability to administer precise dosages.
A problem with breath activated drug delivery is that the dose is triggered on crossing a fixed threshold inspiratory effort. Thus, an inspiration effort may be sufficient to release a metered dose, but the inspiratory flow following the release may not be sufficient to cause the aerosol medication to pass into the desired portion of the patient's airways. Another problem exists with patients whose inspiratory effort is not sufficient to rise above the threshold to trigger the release valve at all. Yet another problem is that the velocity and size of the particles released can vary greatly.
Attempts have been made to solve the patient inspiration synchronization problem. U.S. Pat. No. 4,484,577 refers to using a bidirectional reed whistle to indicate to the patient the maximum rate of inhalation for desired delivery of the drug and flow restrictor to prevent the patient from inhaling too rapidly. U.S. Pat. No. 3,991,304 refers to using biofeedback techniques to train the patient to adopt a desired breathing pattern. U.S. Pat. No. 4,677,975 refers to using audible signals and preselected time delays gated on the detection of inspiratory flow to indicate to the patient when to inhale and exhale, and delivering inhalable material a selected time after the detected onset of flow.
Studies in Byron (ed.), Respiratory Drug Delivery, CRC Press, Inc. (1990); Newman et al., Thorax, 1981, 36:52-55; Newman et al., Thorax, 1980, 35:234; Newman et al., Eur. J. Respir. Dis., 1981, 62:3-21; and Newman et al., Am. Rev. Respir. Dis., 1981, 124:317-320 indicate that during a single breath of an aerosol compound, only about ten percent of the total aerosol material presented is deposited into the lungs and that the location of deposition in the lung depends upon (1) breath parameters such as volume of inspiration, inspiratory flow rate, breath holding prior to expiration, the lung volume at the time the bolus of medication is administered, and expiratory flow rate, (2) the size, shape and density of the aerosol particles (i.e., the medicinal compound, any carrier, and propellant), and (3) the physiological characteristics of the patient. Present devices and methods cannot eliminate these variables and as such cannot control dosage administration.
A problem with existing metered dose inhalers, whether or not breath actuated, is that they are factory preset to deliver a fixed dose at a given particle size distribution. Such devices are not capable of reducing the dose to reflect improvement in the patient's condition, or selecting a maximum desired respirable fraction of the aerosol mist that is suitable for a desired location of delivery of the medication in the particular patient.
Devices for controlling particle size of an aerosol are known. U.S. Pat. No. 4,790,305 refers to controlling the particle size of a metered dose of aerosol for delivery to the walls of small bronchi and bronchioles by filling a first chamber with medication and a second chamber with air such that all of the air is inhaled prior to the inhaling medication, and using flow control orifices to control the flow rate. U.S. Pat. No. 4,926,852 refers to metering a dose of medication into a flow-through chamber that has orifices to limit the flow rate to control particle size. U.S. Pat. No. 4,677,975 refers to a nebulizer device that uses baffles to remove from any aerosol particles above a selected size. U.S. Pat. No. 3,658,059 refers to a baffle that changes the size of an aperture in the passage of the suspension being inhaled to select the quantity and size of suspended particles delivered. A problem with these devices is that they process the aerosol after it is generated and thus are inefficient.
It is well known that pulmonary functions, such as forced expiratory volume in one second, forced vital capacity, and peak expiratory flow rate, can be measured using flow rates and used to (1) diagnose the existence of medical conditions, (2) prescribe medication, and (3) ascertain the efficiency of a drug therapy program. See, for example, U.S. Pat. Nos. 3,991,304 and 4,852,582 and the publications of Newman et al. discussed above wherein volume but not flow rate is measured to determine the point of drug release which is preferred for maximizing the delivery of the drug. Heretofore, these tests have been performed using available spirometers. U.S. Pat. No. 4,852,582 also refers to using a peak flow rate meter to measure changes in peak flow rate before and after administration of a bronchodilator. The results of such tests before and after administration of several different medications are used to evaluate the efficiency of the medications.
Some devices only measure pulmonary function and do not deliver drugs. Devices which do deliver drugs are often too complicated for most patients to use effectively and obtain repeated delivery of a given amount of drug i.e. user error in administration causes significant variability in the amount of drug the patient receives. In other devices the data obtained does not directly effect the operation of the device, i.e. it must be examined and interpreted by a trained medical practitioner to be meaningful.
Attempts have been made to solve many of the above-referred-to problems. However, inconsistent user compliance combined with undesirably large and variable particle size continues to cause problems with obtaining precise dosing.
Nebulizers utilize various means in order to create a fog or mist from an aqueous solution or suspension containing a pharmaceutically active drug. The mist created by the nebulizer device is directed towards the face of the patient and inhaled through the mouth and nose. Nebulizer devices and methodology can be quite useful when the precise dosing of the drug being delivered to the patient is not of particular importance. For example, in some situations the nebulizer creates a mist from an aqueous solution containing a bronchodilator which can be inhaled by the patient until the patient feels some improvement in lung function. When precise dosing is more important the nebulizer device and delivery methodology suffers from many of the disadvantages of metered dose inhaler devices and methodology as described above. In addition, nebulizers are generally large in size and not hand-held, easily transportable devices like MDIs. Large nebulizers can only be used within a fixed location such as the patient's home, the doctor's office and/or hospital. However, a portable nebulizer is taught in published PCT application WO92/11050 incorporated herein by reference. Another nebulizer which uses a high frequency generator to create an aerosol is described in U.S. Pat. No. 3,812,854 issued May 28, 1974. Drug formulations placed in nebulizers are generally diluted prior to delivery. The entire diluted formulation must generally be administered at a single dosing event in order to maintain the desired level of sterility and the nebulizer must be cleaned after use. Yet another disadvantage of nebulizers is that they produce an aerosol which has a distribution of particle sizes not all of which are of appropriate size to reach the targeted areas of the lung. The present invention endeavors to address and solve these and other problems.